When I add a new folder/album to Photos and hit the refresh database button, the new folder imports and all pictures appear as expected.But for several years now, if I add 1 or more photos to an existing album, the only way I can make it/them appear is to delete and recreate the DB. The they import.

first refresh is a "quick" refresh it just looks for new folders, it ignores existing folders it knows about. If you press refresh again within 15seconds it will do a full refresh and check existing folders it knows about.

A long time ago it would always check everything but people with huge movie and music collections couldn't handle the time to check everything.